1. Field of the Invention
The present invention relates to an apparatus and method for suppressing unwanted resonance in an electromanometry system, and in particular to an apparatus and method for matching the characteristic impedance of an electromanometry system.
2. The Prior Art
By the pumping action of the heart a quantity of incompressible blood is intermittently pumped into an elastic reservoir of blood vessels. The resistance of these blood vessels causes a build-up in the arteries of a continuous mean blood pressure with a superimposed fluctuation between a maximum and a minimum pressure, known as systolic and diastolic values, respectively. The heart's pumping action results in the cyclical recurrence of these blood pressures.
Sometimes the life of a patient depends on a continuous, accurate measurement of these cyclical blood pressures. Thus, the use of electromanometry systems for monitoring and recording hemodynamic pressures has become an indispensible technique in many modern hospitals.
In order to transform hemodynamic pressures into recordable electronic waveforms, the interior of a patient's artery may be connected via a liquid-filled catheter to an electrical pressure transducer. In this way, the periodic pulsations of blood are transmitted as pressure pulses through the liquid-filled catheter to the transducer, where they are transformed into recordable waveforms. The output of the transducer may be recorded on an oscilloscope or strip chart, from which it is possible to ascertain a number of parameters, as for example heart rate, duration of systole, and systolic, diastolic and mean pressures. Each of these parameters may provide important diagnostic data about a patient's condition.
One of the problems that is frequently encountered when using an electromanometry system is the problem of resonance. Resonance describes the tendency of a catheter-transducer system to amplify pulsations that are in the region of the system's natural resonant frequency to a much greater degree than pulsations having other frequencies. Resonance in a catheter-transducer system results in a form of distortion known as "ringing" or "harmonic ringing" in the recorded waveforms. This distortion hampers the ability of the electromanometry system to provide the needed diagnostic data in an accurate and reliable form, and limits the range of frequencies over which the system can faithfully reproduce detected hemodynamic pressures.
One of the prior art techniques for damping (i.e. suppressing) unwanted resonance is the use of electrical equilisation circuitry. See, for example, Latimer, K. E., "The Electrical Equalisation of Electromonometry and Phono Cardiography Systems," Bibliotheca Cardiologica 31:20 (1973). Although electrical equalisation may be used to suppress resonance distortion to a desirable degree, the necessary electronic circuitry is often complex and relatively difficult and expensive to implement, particularly in existing electromanometry systems.
Other prior art attempts to suppress unwanted resonance in catheter-transducer systems have relied on series or parallel hydraulic damping techniques. See, for example, Crul, J. F., "Measurement of Arterial Pressure," Acta Anaesthesiologica Scandinavica 6 Suppl. 11:135 (1962); Latimer, K. E., "Extending the Frequency Spectrum of Electromanometry Systems into Audio Frequencies," Bibliotheca Cardiologica, 31:7 (1973); and Latimer, R. D. and Latimer, K. E., "Continuous Flushing Systems," Anaesthesia 29:307 (1974).
Series or parallel hydraulic damping techniques are less expensive and more simple to use than electronic equalisation techniques, and they provide acceptable resonance suppression in many applications. However, to date there has not been devised an apparatus and method for suppressing resonance in catheter-transducer systems which is inexpensive, effective and which provides a wide range of control for use with numerous different types of electromanometry systems. Such an invention is described and claimed herein.